What Causes Red Patch On The Top Of The Mouth?

an elderly neighbor lady friend of mine just recently got over the flu she has noticed recently a red patch on the roof of her mouth and she said it is sore. She has been also eating tomatoes from her garden and strawberries and orange juice. She said her mouthwash burns and that it rubs against her dentures and gives her a painful burning sensation. She needs to wear her dentures to a special event and was wondering if baking soda would help? She has noticed these symptoms for two days. Is there anything she can rinse her mouth with or use to soothe her burning mouth symptoms?